So, mask from divination states that it works like non detection except where "In the case of divination spells that would normally reveal the wearer’s presence, such as see invisibility, the spell works, but the wearer is detected only if the caster succeeds at a caster level check."
This makes sense that it would work on true seeing as well, the caster of true seeing would need to succeed on that check.
My question is how does this affect monsters that have True Seeing as innate abilities but don't have a caster level (ex: Crag Linnorm)?
On page 304 of Bestiary 1, it says:
If no caster level is specified, the caster level is equal to the creature’s Hit Dice. The saving throw (if any) against a spell-like ability is 10 + the level of the spell the ability resembles or duplicates + the creature’s Charisma modifier
I missed that it was Ex, not Sp. Arguably, it can't be countered at all in that case. I'd probably treat it like a Spell-like ability as described above. I haven't seen a clear RAW pointing one way or the other, but the Spell-like abilities thing seems a little clearer (since you're not really dispelling/negating in this case, they just conflict).
Page 221 of the CRB says:
These abilities cannot be disrupted in combat, as spells can, and they generally do not provoke attacks of opportunity. Effects or areas that negate or disrupt magic have no effect on extraordinary abilities. They are not subject to dispelling, and they function normally in an antimagic field. Indeed, extraordinary abilities do not qualify as magical, though they may break the laws of physics.
As described in the rules for Extraordinary abilities, a creature with the True Seeing (Ex) innate ability would not be fooled by a spell such as Mask from Divination. While the True Seeing spell and any creature or class with True Seeing (Sp) would be subject to a CL check, the True Seeing (Ex) ability is not.